our MVAC system, when engaged in most positions of that rotary switch, engages the compressor which presents a load to the engine. There's a mod to switch it off, which you can no doubt do easily since you put the vacuum gauge in so nicely. I'll see if I can dig up the link for you, but it will allow you to disengage the compressor for extra engine power on long uphill climbs, for instance.
I'd bet if you performed the IAC re-calibration with that restrictor plate on, you'd be able to get it idling nice and low. look around on here...I shared it in one of the forums.
As far as the roles of sensors and computer, walk with me:
from after the air filter, the first one you'd encounter as an air molecule on the way to combustion is the combined MAF and IAT sensor that tells the computer the actual weight of air present before the throttle plate (remember 14.7:1 air-fuel ratio is of weights of each). the throttle plate is cracked open a wee bit to allow enough air in for the engine to idle, and the TPS tells the computer what that angle is, so it guesstimates how much air is getting in so it can guesstimate how long each injector stays open based on ~6 lbs/gallon. say the TPS is reporting 11 degrees of opening, divided by 90 is .12222, so 12% of the air in front of the throttle plate is getting past it, so injectors only need to stay on for ~12% of their maximum. into the cylinder, bang, and out past the upstream O2, where the math is checked and it adjusts fuel delivery.
All of this, hundreds of times per minute.
Now, when the ECT sensor reports cold (below 92-95 degrees in Fords), the computer dumps the maximum amount of fuel dictated by engineers in Detroit to run, EPA standards be damned, because the Catalytic converter will clean that up. And this is where the IAC valve comes in: it sneaks in more and more air past the "closed" throttle to balance out what the injectors are firing into the chambers until the upstream O2 says "woah, enough, this engine has to get up to operating temperature, and it does that by burning fuel." Once the IAC closes fully, your engine will be rapidly approaching proper temp and the coolant system will start to operate to keep it there and the computer will enter closed loop with lean fuel delivery.
Have you noticed that the amount of fuel delivered depends on what the computer THINKS the engine needs based on how much air it THINKS it's taking in until that gets verified by the O2 sensors on the way out, well after the fact? Bass-ackwards, isn't it?
This is where the groove comes in. In the videos Ron talks about a waveform of little balls of air in the intake manifold. in combining fuel with these balls of air better than just the factory air stream, we're creating a fuel charge in the cylinders more rich than the computer is accustomed to seeing, and when it sees that at the O2 sensor, it trims delivery way back from where the throttle plate tells it it should be, until it's re-learned what Detroit taught it.
Here's the rub: 14.7 masses of air to 1 mass of gasoline is for sea-level, open air combustion...an engine cylinder is a warmer place with much more air pressure than at sea level on this planet, and that has an effect on how much air and fuel are needed to create a powerful enough instantaneous explosion to move the piston with the most possible force. Those videos Ron made show air-fuel ratios up toward or maybe even above 20:1, post groove, with super low emissions. Right, we've super-fine tuned a mechanism to operate to our specifications, not to the wide range of possibilities that auto manufacturers have to take into consideration.